The present invention relates to a wavelength stabilized semiconductor laser.
The stabilization of a wavelength in a semiconductor laser is permitted by endowing laser radiation with distributed feedback based on a refractive index change or/and distributed feedback based on a gain change. At the very early stage at which a distributed feedback (abbreviated to `DFB`) laserd was realized, a double heterostructure was fabricated in such a way that a GaAs layer, being a laser active layer, was directly provided with a grating structure and was sandwiched between two semiconductor layers (p-type GaAlAs and n-type GaAlAs) having wider band gaps and having conductivity types opposite to each aother. With a laser of this type, since refractive index of GaAs is higher than that of GaAlAs, coupling based on the index change arises. Further, since the laser active layer itself changes in thickness periodically in the propagation direction of the laser radiation, coupling based on the gain change arises. In this system, the active layer having a high density of carriers is directly corrugated, and the nonradiative recombination of the carriers occurs due to lattice damages at the corrugated heterointerface. For this reason, lasing took place only at low temperatures of 80.degree.-150.degree. K. (Nakamura et al., IEEE J. of Quantum Electronics, Vol. QE-11, No. 7, 436 (1975)). In order to solve this problem, a method in which a corrugated part and a laser active layer were separated (separate-confinement heterostructure: SCH) was adopted (H. C. Casey et al., Appl. Phys. Lett. 27, 142 (1975) and K. Aiki et al., Appl. Phys. Lett. 27, 145 (1975)). Thus, the injected carriers were prevented from causing the nonradiative recombination ascribable to the damages near the corrugation, and the lasing became possible at room temperatures. With this laser, the gain is uniform in the propagation direction of laser radiation, so the coupling based on the gain does not arise, and only the coupling based on the modulation of the refractive index arises. In this case, as regards the longitudinal mode of the laser, two modes near the Bragg frequency have equal gains, so that when reflected light has entered the laser, the longitudinal mode changes to cause noise. In order to prevent such influence of the reflected light upon the stability of the lasing, the method of the distributed feedback may be done by gain coupling, and the longitudinal mode of the laser may be brought into agreement with the Bragg frequency itself (H. Kogelnik, J. of Appl. Phys. 43, 2327 (1972)).
To sum up, in order to obtain a frequency-stabilized laser which involves little noise attributed to reflected feedback light from surroundings such as an optical fiber and which operates stably at the toom temperature, it is required that the distributed feedback of the gain be given to the laser active region itself and that a method and a structure for preventing the occurrence of the lattice damages be developed.
Such a structure is described in Japanese Patent Application Laid-open No. 50-114186. In order to obtain the distributed feedback of the gain, a double hetero-region including a laser active layer is removed by ion milling or chemical etching, whereupon GaAlAs is buried by liquid phase epitaxy (LPE). It is known, however, that the etching by the ion milling causes a large number of damages. With the chemical etching, slits more than 1 .mu.m deep cannot be formed at pitches of several thousand .ANG., and the desired results discussed above can not be achieved.
Japanese Patent Appliction Laid-open No. 51-71684 teaches a distributed feedback laser wherein GaAs doped with Cr is periodically disposed between the p-side electrode and p-type GaAlAs cladding layer of the laser, to endow the injection current density in the propagation direction of laser radiation with a periodicity. However, a cladding layer 1-1.5 .mu.m thick is required for confining the laser radition in the thickness direction of a crystal with low loss, and the current density endowed with the distributed feedback becomes uniform in the active layer due to the spread of current.
There has been reported in Japanese Patent Application Laid-open No. 52-45888, a method wherein, in order to prevent this effect, protons etc. are periodically implanted directly in an active layer so as to establish a distributed current density. However, a laser with the protons implanted in the active layer large number of damages and has not been put into practical use even for controlling the transverse mod of the laser.